Tvrtko I, in full Tvrtko Kotromanić (born c. 1338—died 1391) probably the greatest ruler of Bosnia, ruling as Bosnian ban (provincial lord, subservient to the king of Hungary) from 1353 and king of the Serbs and Bosnia from 1377.

In 1363 Tvrtko commenced war with King Louis I of Hungary, but afterward Louis helped him regain power following a revolution in Bosnia. At Bileća in 1388 Tvrtko’s forces halted an Ottoman Turkish invasion. In 1390 he extended his power into Croatia and to the Dalmatian islands in the Adriatic Sea.

...war, Hungarian interference, and the threat of invasion by the Turkish Ottoman Empire. Ottoman armies began raiding Serbia in the 1380s and crossed into Bosnian-ruled Hum (Herzegovina) in 1388. King Tvrtko I sent a large force to fight against them alongside the Serbian army at the Battle of Kosovo Polje in the following year. Tvrtko’s successor, Stjepan Ostoja, struggled for possession of the...

...it hoped would be a neutral symbol from the far past. It chose the coat of arms used by King Stjepan Tomaš (reigned 1443–61), which it attributed to the greatest medieval Bosnian ruler, Tvrtko I (1377–91). That blue shield with a white diagonal stripe separating six golden fleurs-de-lis was placed in the centre of a white flag. The artistic rendition, officially adopted on May...